Desulfurization of coal

ABSTRACT

A method for desulfurizing coal by reacting sulfur-containing bituminous coal with hydrogen in the presence of a hydrogen sulfide &#39;&#39;&#39;&#39;getter&#39;&#39;&#39;&#39; at a temperature about 600-800* F.

v I United States Patent [151 3,640,016 Lee et al. Feb. 8, 1972 [54]DESULFURIZATION 0F COAL [56] References Cited [72] Inventors: BernardShing-shu Lee, Lincolnwood; UNITED STATES PATENTS Frank C. Schora, Jr.,Palatine, both of ill. 2,726,148 12/1955 McKinley et al ..44/1 1 AsslsnwInstitute of Gas Techlmhgy 2,824,047 2/1958 Gorin et al ..201/17 22Filed; 2 19 9 3,130,133 4/1964 Loevenstein .23/209.9 X PP 811,654 OTHERPUBLICATIONS Guntermann et al. Chemical Abstracts" Vol 62, Apr. RelatedUS. Application Data Q JVQ 89172 d [63] Continuation-impart of Ser. No.579,923, Sept. I6,

1966, abandoned, Primary ExaminerEdward J. Meros Attorney-Molinare,Allegretti, Newitt and Witcoff [52] US. Cl ..44/1, 23/209.9, 23/225,

2 [57] ABSTRACT [51] Int. Cl. ..Cl0l 5/00 5s 1 Field of Search..23/209.9, 66, 131, 225; A method for desulfuflzmg coal by reactmgsulfur-contammg 201/17; 44/1 bituminous coal with hydrogen in thepresence of a hydrogen sulfide getter at a temperature about 600-800 F.

7 Claims, 1 Drawing Figure DESULFURIZATION or COAL CROSS-REFERENCES TORELATED APPLICATION This is a continuation-in-part of our parentapplication, Ser. No. 579,923, filed Sept. 16, 1966, for Desulfurizationof Coal," now abandoned. BACKGROUND OF THE INVEN- TION This inventionrelates to a novel method for desulfurizing coal at low temperatures toproduce desulfurized coal useful as fuel, and, in addition, elementalsulfur as byproduct. Desulfurized coal made by our method provides afuel product which greatly reduces the emission of air pollutants andincreases the market for coal, especially high-sulfur coal.

By way of background, the sulfur content in coal can be broadly dividedinto three classes: pyritic, organic and sulfate. The last named occursonly in weathered coal, as CaSO and can be neglected essentially whendealing with freshly mined coal. As total sulfur content of the coalincreases, the percentage of sulfur as pyritic sulfur also increases. Inhigh-sulfur coals, pyritic sulfur constitutes the bulk of the totalsulfur. In a desulfurization scheme by hydrogenation such as ours,chemical equilibrium calculations indicate that the organic sulfur inthe coal should be more readily hydrogenated than the pyritic sulfur.Thus when dealing with high-sulfur coals as candidates fordesulfurization treatment, one must cope with pyritic sulfur as thechief contributor to total sulfur.

Pyrites are typically represented by the formula FeS x being positive.The removal of the x portion of the sulfur is relatively easy. In fact,certain pyrite ore gives off the first sulfur directly upon heating aselemental sulfur vapor. Thus for process considerations, attention isfocused on the FeS portion.

Various thermodynamic studies of the reaction:

FeS+H Fe+H S 1 indicate that even at temperatures as high as 1,300 E,the equilibrium partial pressure ratio of hydrogen sulfide to hydrogenare still so low that a tremendous recycle of hydrogen is necessary. Atequilibrium conversion some 400,000 s.c.f. of hydrogen have to berecirculated to reduce the sulfur in one ton of coal from 4 to 0.5percent. Subsequent removal of H 8 from the hydrogen stream isuneconomical even when considering byproduct sulfur credit. To go tohigher temperatures would completely devolatilize the coal withoutreducing the hydrogen recycle appreciably.

There have been attempts to desulfurize coal by oxidation. However, suchprocesses consume a substantial portion of the coal without reducing thesulfur content sufficiently. Altematively, desulfurization by hydrogenreduction alone, as pointed out above, has consistently been limited byequilibrium conversion even at high temperatures as well as bytremendous hydrogen recycle problems.

There have also been proposed desulfurization processes wherein ammoniais added to sulfur-containing coal and the sulfur converted to gaseousproducts by nascent hydrogen produced in the decomposition of theammonia. However, such processes still suffer from the adverse H -H Sequilibrium considerations above noted.

This problem has been further complicated by the fact that most of thecoal having a high sulfur content, which therefore requires sometreatment to remove a portion of the sulfur therefrom, is a bituminousvariety which is also known as the caking coal. As indicated in anAmerican Society of Mechanical Engineers publication, No. 66-PWR-3,entitled The Search for Low-Sulfur Coal by Harry Perry and Joseph A.DeCarlo, engineers with the Bureau of Mines, U.S. Department ofInterior, on the coal reserves in the United States, practically all ofthe coal containing more than 1.5 percent sulfur by weight are of thebituminous or caking coal variety. Unfortunately, some of the availablemethods for desulfurization of carbonaceous solid fuels are specificallynot applicable to the caking type of coal. Thus, there is a present needfor a process which is capable of desulfurizing the type of coal whichcontains large amounts of sulfur.

SUMMARY OF THE INVENTION AND DESCRIPTION OF THE PREFERRED EMBODIMENT Wehave now invented a new method for desulfurizing caking coal whichspecifically removes the equilibrium limitation in reaction (1) byhydrogenating in the presence of a getter. The getter chemicallycombines with the hydrogen sulfide formed. Introducing the getter"strongly shifts the equilibrium conversion toward greater hydrogenutilization by per-. mitting more desulfurization to take place.Examples of cheap and effective getter" is lime, calcined dolomite, orother alkaline earth metal oxides. In the following discussions lime isused as the example with the understanding that the others are just assuitable.

It is thus the object of this invention to provide a novel process fordesulfurizing coal by hydrogcnating sulfur in the coal in the presenceof a getter for H 8 whereby the unfavorable equilibrium condition inconverting FeS to H 5 is overcome.

Broadly speaking, our invention is based on the fact that the hydrogenreduction of FeS by itself proceeds according to reaction (I) which at800 F., for example, permits a ratio of hydrogen sulfide to hydrogen of0.00004 at equilibrium, requiring a recycle of 99.996 percent of thehydrogen. With a getter such as CaO, the desulfurization proceedsaccording to:

CaO-H'I l-FeS rt CaS+Fe+H O 2 which at 800 F., permits a ratio of steamto hydrogen of 3 at equilibrium, requiring a recycle of only 25 percentof the hydrogen.

In the 600800 F. range, at the higher temperatures by colrtrolling thepartial pressure of water vapor, operation is according to equation (2)and no hydrated lime is formed according to (3).

At low temperatures, equations (2) and (3 )yield:

2CaO+FeS+H CaS-l-Ca( OH) +Fe 4 At intermediate temperatures, the surfaceof the lime particles may be reacted with water vapor to form hydratedlime. Then further reaction is by:

Ca(OH) -l-FeS+H CaS+Fel-2H O s; This equation permits asteam-to-hydrogen partial pressure ratio of 2.5 at 800 F. Thus, in the600 to 800 F. range, it is possible to achieve good steam-to-hydrogenpartial pressure ratios so that the hydrogen can be recycled aftersimple condensation of the water. In this temperature range,devolatilization of the coal would be limited, and the desulfurizedproduct would have similar combustion characteristics as the raw coal.

From an economic standpoint, it is necessary to reuse the lime or otheralkaline earth oxide getter material. To regenerate lime, the CaSproduced may by hydrolyzed with steam as follows:

CaS+2H O Ca(OH) +H S (6) With steam, the above reaction proceedssufi'rciently to the right at low temperatures for feasible operations.To increase the rate of reaction Equation (6), the preferred method ofhydrolysis of CaScan be accomplished in the presence of carbon dioxideaccording to:

CaS+H O+CO CaCO +H S 7 In either case, the H 8 produced can be reactedto form elemental sul'fur by processes well known in the art. Forexample, the Claus process operates on the overall reaction:

2H S-l0 2S+2H O (3) The Ca(OH) or Ca CO produced can be dehydrated orcalcined, respectively, to produce lime for recycle.

The drawing shows a schematic flow diagram of one embodiment of ourmethod. Other arrangements of the equipment shown in the drawing will beapparent to those skilled in the art. Raw coal from storage is dried,crushed and sized in suitable particle size reduction equipment 1 toobtain a particle size suitable for fluidization, typically minusone-fourth inch. Particle size is not critical but should be chosendepending upon reactor volume and configuration and gas flow rates.Limestone or other source of alkaline earth metal oxide material, suchas dolomite, is also dried, crushed and sized in suitable apparatus 2 toapproximately the same particle size as the coal and fed to the calciner3 to produce the oxide. The particle size reduction equipment showndiagrammatically at l and 2 and calciner 3 form no part of the inventionper se and may be conventional equipment well in the art.

The coal and lime are then fed into the desulfurizer 4 which may be areactor of any conventional type having a reaction chamber and means toinject gas into it.

A fluidized bed may be used and would be particularly advantageous forgood temperature control. The material in the fluidized bed may comprisefine coal particles with coarse particles of lime raining down throughthe bed, thus permitting subsequent separation of lime. and coalparticles by screening. An altemative is a fluidized bed of coal andlime of essentially the same size, the mixture being separated later bydifferences in density. A third possible arrangement is a fluidizedpacked bed" wherein small coal particles are fluidized in theinterstices of a fixed bed of large pieces of lime. Broadly speaking,any reactor arrangement wherein the coal and lime particles are placedin a relatively close degree of juxtaposition permitting hydrogen gas toflow therethrough can be used in our method. The process is effected atsubstantially atmospheric pressure and at temperatures between about 600and 800 F.

Steam-laden hydrogen gas exiting from the reactor is directed throughheat exchanger 5, and condenser 6, to remove water vapor and thenrecycled with makeup hydrogen from supply to the desulfurizer.

The reacted lime-coal mixture is removed from the desulfurizer and thecoal and lime fractions are separated in a separator device 7 bytechniques, well known in the art, which utilize either the size or thedensity difference between the materials. The unreacted lime-CaS mixtureis then regenerated in a regenerator 8, wherein steam and CO are fedthrough the mixture to effect reactions (6) or (7) as above described.The resultant H 8 gas is then sent to sulfur recovery 9, where e.g.elemental sulfur is produced by well-known techniques such as the Clausprocess.

The regenerated calcium carbonate from 8 is then fed to the calciner 3,along with makeup limestone feed. The carbon dioxide from 3 is used asfeed for the lime regenerator.

Although the above description of our method discusses use of lime orcalcium oxide, it should be understood that other alkaline earth metaloxides or oresv containing alkaline earth metal oxides are equallyeffective. For example, dolomite which, as well known in the art, is amixture of calcium carbonate and. magnesium carbonate is a particularlyuseful material when calcined to produce CaO-MgO.

The following pilot plant data shows a specific embodiment of ourinvention which is intended in no way to limit the scope thereof:

EXAMPLE 1 A mixture containing equal volumes of coal and calcinedlimestone in the size range of minus 16 to plus 80 mesh was reacted at750 F. with hydrogen in a fluidized bed reactor. The reactor was a2-inch diameter stainless steel cylinder about 6 feet long. Flow rate ofhydrogen through the coallimestone mixture was 53 s.c.f./hour (1.5linear feet/sec). The coal employed as the starting material contained1.92 percent pyritic sulfur, 1.78 percent organic sulfur and 0.18percent sulfate sulfur. All percentages are based on the weight of coal.Analysis of the coal after the treatment showed substantially no pyriticsulfur. The organic sulfur content was reduced by about 34 percent.Although the sulfate content was also substantially reduced, this itemis not particularly significant in view of the low percentage present inthe starting material. No hydrogen sulfide was detected in the effluentgas, indicating that substantially all hydrogen sulfide has reacted withthe lime. As indicated at the beginning of this application, the pyriticsulfur is the chief contributor to the total sulfur content of coalswhich are to be treated for desulfurization. ln that respect, the methodof the presentinvention is especially effective.

EXAMPLE 2 A sample of calcined limestone which had previously beenreacted with hydrogen sulfide until it contained 15.6 percent sulfur byweight was regenerated as follows: a sample of such sulfur-containinglimestone was placed in an Erlenmeyer flask. The sample was then coveredwith water. Carbon dioxide was bubbled through the mixture for 3 hoursand H 8 was evolved in accordance with reaction (7) above.

EXAMPLE 3 A sample of a caking bituminous coal containing 3.51 percentby weight of sulfur was treated by passing hydrogen over the sample atatmospheric pressure for 1 hour at a temperature between 700 and 800 F.,and then for 2 hours at 800 F. The sulfur content of the coal after thetreatment was found to be about 3.06 percent by weight. This exampleshows that the presence of the getter is necessary in order to reducethe sulfur content of the coal to an acceptable level.

The invention has been described in detail with reference to particularand preferred embodiments thereof, but it will be understood thatvariations and modifications can be made within the spirit and scope ofthe invention as described hereinabove and as defined in the appendedclaims.

What is claimed is:

l. A method of desulfurizing caking coal without substantial caking,devolatilization and conversion to coke or char, which comprises thesteps of:

a. mixing a solid getter material with sulfur-containing caking coal,said getter and said caking coal being in particulate form,

b. passing hydrogen gas through said particulate getter-caking coalmixture to form a fluidized bed of said particulate mixtureand tomaintain the particulate integrity thereof,

c. maintaining said mixture at a temperature in the range of from about600 to 800 F. and at substantially atmospheric pressure, to form anongaseous sulfide by reactions with said getter and said hydrogen, and

d. separating said nongaseous sulfide and unreacted getter material fromsaid mixture to recover therefrom particulate, uncaked coal having asubstantial amount of sulfur removed therefrom without substantialdevolatilization.

2. Method of claim 1 wherein said solid getter material is an alkalineearth metal oxide or a compound of alkaline earth metal oxides.

3. Method of claim 2 wherein said solid getter material is calcineddolomite.

4. Method of claim 1 wherein coarse particles of solid getter materialare dropped through a fluidized bed of coal particles, thereby keepingthe solid material and coal particles separate.

5. Method of claim 1 wherein said fluidized bed is a fluidized packedbed with small coal particles being fluidized in the interstices of afixed bed of larger solid getter material particles.

6. Method of claim 1 which includes the added steps of:

a. regenerating getter material from said nongaseous sulfide andrecovering a sulfur-containing byproduct, and

b. recycling said regenerated getter material to said mixing step.

7. Method of claim 6 wherein said solid getter material is an alkalineearth metal oxide or a compound of alkaline earth metal oxides, andwherein said regeneration includes the steps of:

a. reacting said nongaseous sulfide with carbon dioxide and water toproduce hydrogen sulfide-rich gas and regenerated solid getter material,

b. recovering elemental sulfur by oxidation of said hydrogensulfide-rich gas,

c. calcining said regenerated solid getter material to produce oxide andcarbon dioxide, and

d. cycling said carlzion dioxide to said step of reacting saidnongaseous sulfide.

2. Method of claim 1 wherein said solid getter material is an alkalineearth metal oxide or a compound of alkaline earth metal oxides. 3.Method of claim 2 wherein said solid getter material is calcineddolomite.
 4. Method of claim 1 wherein coarse particles of solid gettermaterial are dropped through a fluidized bed of coal particles, therebykeeping the solid material and coal particles separate.
 5. Method ofclaim 1 wherein said fluidized bed is a fluidized packed bed with smallcoal particles being fluidized in the interstices of a fixed bed oflarger solid getter material particles.
 6. Method of claim 1 whichincludes the added steps of: a. regenerating getter material from saidnongaseous sulfide and recovering a sulfur-containing byproduct, and b.recycling said regenerated getter material to said mixing step. 7.Method of claim 6 wherein said solid getter material is an alkalineearth metal oxide or a compound of alkaline earth metal oxides, andwherein said regeneration includes the steps of: a. reacting saidnongaseous sulfide with carbon dioxide and water to produce hydrogensulfide-rich gas and regenerated solid getter material, b. recoveringelemental sulfur by oxidation of said hydrogen sulfide-rich gas, c.calcining said regenerated solid getter material to produce oxide andcarbon dioxide, and d. cycling said carbon dioxide to said step ofreacting said nongaseous sulfide.